1. Field of the Invention
The present invention relates to a receiving apparatus, a receiving method, and a receiving program, and, more particularly, to a receiving apparatus, a receiving method, and a receiving program that are configured to early execute the decoding of an intended PLP (Physical Layer Pipe) in an OFDM (Orthogonal Frequency Division Multiplexing) signal of DVB-T2 (Digital Video Broadcasting-Terrestrial 2).
2. Description of the Related Art
Terrestrial broadcasting and so on use OFDM for data (or signal) modulating.
With OFDM, many orthogonal subcarriers are arranged in a transmission band and digital modulation, such as PSK (Phase Shift Keying) or QAM (Quadrature Amplitude Modulation), is executed to allocate data to the amplitude and phase of each of these subcarriers.
In order to divide the transmission band by many subcarriers, the band for each subcarrier (one wave) is narrow and the modulation speed is low, but the total transmission speed (of all subcarriers) remains substantially the same as the related-art modulating.
As described above, in OFDM, data is allocated to two or more subcarriers, so that the modulation can be executed by executing IFFT (Inverse Fast Fourier Transform). And the demodulation of an OFDM signal obtained as a result of the modulation can be executed by FFT (Fast Fourier Transform).
Therefore, a transmission apparatus configured to transmit OFDM signals can be configured by use of an IFFT computation circuit and a receiving apparatus configured to receive OFDM signals can be configured by use of an FFT computation circuit.
Further, with OFDM, a signal section called a guard interval is arranged to enhance the resistance against multipaths. In addition, with OFDM, a pilot signal that is a known signal (known on the side of the receiving apparatus) is discretely inserted in the direction of time or in the direction of frequency to be used by the receiving apparatus for synchronization, the estimation of transmission path characteristics, and so on.
Because OFDM has a high resistance against multipaths, OFDM is used by the terrestrial digital broadcasting and so on that are susceptible to multipath interference. The terrestrial digital broadcasting using OFDM includes DVB-T and ISDB-T (Integrated Services Digital Broadcasting-Terrestrial), for example.
With OFDM, data is transmitted on an OFDM symbol unit basis.
An OFDM symbol is generally configured by a valid symbol that is a signal interval in which IFFT is executed at the time of modulation and a guard interval with a partial waveform of the last half of this valid symbol copied to the beginning of the valid symbol without change.
Thus, arranging a guard interval at the beginning of each OFDM symbol allows the enhancement of the multipath resistance.
It should be noted that the terrestrial digital broadcasting standard based on OFDM defines a unit called a frame (an OFDM transmission frame) configured by two or more OFDM symbols and data transmission is executed on a frame basis.
In the receiving apparatus configured to receive OFDM signals as described above executes OFDM signal digital quadrature demodulation by use of the carrier of an OFDM signal.
It should be noted, however, that the OFDM signal carrier for use in digital quadrature demodulation in a receiving apparatus does not generally match an OFDM signal carrier for use in a transmission apparatus that transmits OFDM signals, including an error. To be more specific, the frequency of OFDM signal carrier for use in digital quadrature demodulation is offset from the center frequency of an OFDM signal (an IF (Intermediate Frequency) signal thereof) received by the receiving apparatus.
Consequently, the receiving apparatus executes carrier offset detection processing for detecting a carrier offset that is an error of the carrier of an OFDM signal for use in digital quadrature demodulation and correction (offset correction) processing for correcting the OFDM signal so as to eliminate the offset of the carrier by following the carrier offset.
It should be noted here that DVB-T2 (the second-generation European terrestrial digital broadcasting standard) is being drawn up for a terrestrial digital broadcast standard that uses OFDM.
For DVB-T2, refer to so-called DVB Blue Book A122 (“Frame structure channel coding and modulation for a second generation digital terrestrial television broadcasting system (DVB-T2)”, DVB Document A122 June 2008).
DVB-T2 (the Blue Book thereof) defines a frame called a T2 frame. Data is transmitted in the unit of this T2 frame.
A T2 frame has two types of preamble signals called P1 and P2. These preamble signals contain the information necessary for the processing, such as demodulation of OFDM signals.
The P1 symbol is a symbol for transmitting P1 signaling. The P1 signaling includes a transmission type and basic transmission parameters.
To be more specific, P1 signaling (P1) contains parameters S1, S2, and so on. The parameters S1 and S2 are indicative in which of the schemes, SISO (Single Input, Single Output (meaning one transmitting and one receiving antenna)) and MISO (Multiple Input, Single Output (meaning multiple transmitting antennas but one receiving antenna)) P2 is transmitted, an FFT size (the number of samples (or symbols) subject to one session of FFT computation) for FFT computation of P2, and so on.
Therefore, the demodulation of P2 desires the decoding of a bit train corresponding to parameters S1 and S2 by orthogonally demodulating P1 for example.
The P2 symbol is a symbol for transmitting L1 pre-signaling and L1 post-signaling.
The L1 pre-signaling includes information for a receiving apparatus configured to receive a T2 frame to receive and decode the L1 post-signaling. The L1 post-signaling includes parameters necessary for a receiving apparatus to access the physical layer (the layer pipes thereof).
It should be noted that the T2 frame can have one to 16 OFDM symbol P2 preamble signals.
In addition, P1 and P2 each include pilot signals that are known signals. Namely, with P1, pilot signals are arranged on the subcarrier at a non-periodical position and, with P2, pilot signals are arranged on the subcarrier at a periodical position. Of the pilot signals, one that is periodically arranged for each predetermined number of subcarriers (or symbols) is called an SP (Scattered Pilot) and the other that is arranged on the subcarriers of the same frequency is called a CP (Continual Pilot).
Further, with the receiving apparatus, the FFT computation of OFDM signal is executed for each OFDM symbol. In DVB-T2, the number of symbols (or subcarriers) configuring one OFDM symbol, namely, FFT sizes are of six types, 1K, 2K, 4K, 8K, 16K, and 32K.
It should be noted here that a subcarrier interval of an OFDM symbol is in inversely proportional to the FFT size of the OFDM symbol. Therefore, the specification of the FFT size in DVB-T2 is equivalent to the specification of subcarrier intervals.
Further, DVB-T2 specifies that, for the OFDM symbols of P1, only 1K is used of the above-mentioned six types of FFT sizes and, for the other OFDM symbols, namely, P2 and others, specifies that any of the above-mentioned six types of FFT size is usable.
Consequently, for the OFDM symbols of P1, only the subcarrier having the widest subcarrier interval (the interval corresponding to the FFT size of 1K) among the subcarrier intervals specified by DVB-T2 is used.
For the OFDM symbols of P2 and others, namely the OFDM symbols of other than P1, that is, the OFDM symbols of P2, and the OFDM symbols of data (Normal), the subcarrier having any of the subcarrier intervals other than the widest subcarrier interval specified by DVB-T2 (namely, the intervals corresponding to the FFT sizes of 2K, 4K, 8K, 16K, and 32K) is usable in addition to the widest subcarrier interval.
It should be noted here that the OFDM signal of P1 has 1K (=1024) symbols as valid symbols.
The OFDM signal of P1 has a cyclic structure in which signal B1′ obtained by frequency-shifting part B1 on the beginning side of a valid symbol is copied to the front side of the valid symbol and signal B2′ obtained by frequency-shifting part B2 that is the remaining part of the valid symbol is copied to the rear side of the valid symbol.
The OFDM signal of P1 has 853 subcarriers as valid subcarriers. Of these 853 subcarriers, DVB-T2 locates information to 384 subcarriers.
The DVB-T2 Implementation Guidelines (ETSI TR 102 831: IG) describes that, if the transmission band for transmitting OFDM signals is 8 MHz for example, a “coarse” carrier frequency offset with a maximum accuracy of +/−500 KHz can be estimated by use of the correlation between the locations of the above-mentioned 384 subcarriers according to P1.
In addition, the Implementation Guidelines mentioned above describes that a “fine” carrier frequency offset with an accuracy of +/−0.5 subcarrier interval can be estimated by the cyclic structure of P1.
It should be noted here that, with a receiving apparatus configured to receive OFDM signals of DVB-T2, the demodulation of P1 signaling and the estimation of guard interval length are executed in the T2 frame in which P1 was first detected at the time of so-called channel scan.
Next, the receiving apparatus recognizes the FFT size of P2, thereby enabling the detection of the start position of the FFT computation of P2 of the next T2 frame. Then, the receiving apparatus executes the FFT computation of P2 to enable the decoding of the L1 pre-signaling included in P2 and the decoding of an intended PLP (Physical Layer Pipe) via the decoding of the L1 post-signaling.
Consequently, MPEG streams can be captured as data of a predetermined program, for example.